1. Field of the Invention
The present invention relates to an antenna device, a wireless communication apparatus, a control method of controlling the wireless communication apparatus, a computer program product therefor, and a computer-readable storage medium therefor. More particularly, it relates to an antenna device and the like that transmit and receive at the same time an signal according to a multi-input and multi-output (MIMO) communication system using any plural flat antennas.
2. Description of Related Art
Recently, a wireless communication function has been often implemented in an information processing apparatus such as a personal computer, a mobile phone, a communication terminal such as a personal digital assistance, and any various kinds of consumer appliances such as an audio instrument, video equipment, a camera, a printer, and an entertainment robot. Further, such the wireless communication function has been often implemented in not only the electronics but also an access point for a wireless local area network (LAN) and a so-called accessory card of small size such as a card specified by personal computer memory card international association (PCMCIA), a compact flash card (trademark), and a mini peripheral component interconnection (PCI) card. Any wireless card module having such the wireless communication function and a storage function has been adapted.
A residential wireless LAN has particularly used institute of electrical and electronics engineers (IEEE) 802.11a that utilizes a carrier frequency with a band of 5.2 Ghz, and IEEE802.11b/g that utilizes a carrier frequency with a band of 2.4 Ghz. Such these IEEE802.11a and IEEE802.11b/g have data transmission rate of 54 Mbps but any research and development for wireless schemes have been recently performed to implement any faster data transmission rate, accompanying an increase in an amount of data (information) to be dealt with.
The MIMO communication system has been proposed as remarkable one of such the wireless schemes that can perform any faster data transmission rate. In the MIMO communication system, any wireless communication can be performed utilizing any spatial multipath propagation channels by using plural antennas. IEEE 802.11a and IEEE802.11g that have been used in the wireless LAN at the present use as a modulation system an orthogonal frequency division multiplexing (OFDM). Acceptance of OFDM causes any transmission by each sub carrier to be almost considered as flat fading. This allows the MIMO propagation channels to be represented as a simple model to some extent in a relative broadband communication system such as IEEE802.11a and IEEE802.11g, thereby being enabling the MIMO communication system to be implemented in any actual wireless applications.
Japanese Patent Application Publication No. 2002-280826 discloses a wireless communication apparatus having an antenna of such the MIMO communication system. The wireless communication apparatus has a cluster of antenna that is composed of n antennas. The antenna can transmit and receive signals each having a relative low correlation therebetween and has many antenna ports each being coupled to a signal-processing device. The cluster of antennas operates within a frequency band having maximum frequency and at least a pair of the antenna ports is placed in a volume of space whose longest linear dimension is λ/3 or less. Such the antenna arrangement enables the cluster of antennas that can transmit and receive the non-correlated signals at the same time in the MIMO communication system, which is applicable to a mobile wireless terminal device, to be provided.
Japanese Patent Application Publication No. 2002-290148 discloses an antenna array. The antenna array has groups of antennas where each group includes two pairs of antennas. Each of the pairs of antennas in a group contains orthogonally polarized antennas. An antenna circuitry is coupled to the antenna groups to enable the antenna array to operate in a beam forming/steering mode, a diversity mode or a MIMO mode or any combination thereof. Such the antenna array arrangement enables to be provided the antenna array that can operate under the beam forming/steering mode, the diversity mode or the MIMO mode concerning any transmitting and receiving signals without any additional antennas.
Japanese Patent Application Publication No. 2003-204295 discloses a diversity system, a base apparatus, and a diversity control method. The diversity system has plural antenna elements, so that multiple coming electronic waves can be separated into multiple paths for each angle based on the receiving signals received by these antenna elements. By measuring a mutual relationship between the separated paths, plural transmission directions for a transmission diversity can be determined based on the measured mutual relationship. Any transmission weights of the antenna elements for transmitting the signals towards the determined transmission directions can be then calculated. Such the system configuration prevents any fading to enable any attenuation in receiving signals to be improved and prevents the signals from being interfered with each other.
FIG. 1 illustrates a configuration of an MIMO communication system relative to a related art when propagating a signal. The MIMO communication system illustrated in FIG. 1 has a transmission unit 2a in a transmitting terminal 2. The transmission unit 2a has m transmitting antennas TX (m is an integer). A receiving terminal 4 has a reception unit 4a. The reception unit 4a has n receiving antennas RX (n is an integer). The transmitting terminal 2 communicates to the receiving terminal 4 using plural radio wave propagation channels 3.
For example, the transmission unit 2a receives a transmitting signal, performs any spatial multiple encoding thereon to modulate it, and emits (or radiates) any radio waves on the modulated transmitting signal into the radio wave propagation channels 3 by means of m transmitting antennas TX. The receiving terminal 4 receives any coming radio waves through the radio wave propagation channels 3 by means of n receiving antennas RX. The reception unit 4a receives the received radio waves and performs any time-space decoding on it to demodulate and output the receiving signal.
It is to be noted that as demodulation scheme in the MIMO, any various kinds of demodulation schemes have been proposed as following: a demodulation scheme wherein a transmission side knows no channel information and only the reception side performs any linear calculation to demodulate it, for example, zero forcing, vertical bell labs layered space-time (V-BLAST); a demodulation scheme wherein a non-linear calculation is performed and demodulated, for example, so-called maximum likelihood detection (MLD); a demodulation scheme wherein two series of the transmitting signals have any orthogonality to each other by space time block coding (STBC); a demodulation scheme wherein the transmission side has already known any channel information and suitable power distribution corresponding thereto and a phase vector synthesis are performed to transmit and receive the signals, for example, an Eigen-mode MIMO; and the like.
In this MIMO communication system, as a principle, a data transmission rate is linearly increased in proportion to the increased numbers of antennas. Such the MIMO communication system uses any radio wave propagation environment, namely, any multipath environment in which reflection, scatter, diffraction, screening of the signals to be received and transmitted are intermingled in the radio wave propagation channels 3. Accompanying the above, a spatial correlation property thereof has any serious influence on its information transmission faculty, so that in the MIMO communication system, a method of reducing interference between the signals as broadly as possible has been proposed until now.
FIG. 2 illustrates an example of attachment of antenna for linear polarization that is used in the MIMO communication system. According to the antenna attachment for linear polarization shown in FIG. 2, a vertical polarization antenna 5a and a horizontal polarization antenna 5b are attached to a set box 5 or the like. For example, the vertical polarization antenna 5a and the horizontal polarization antenna 5b are orthogonally arranged.
FIG. 3 illustrates an example of attachment of antenna for circular polarization that is used in the MIMO communication system. According to the antenna attachment for circular polarization shown in FIG. 3, a right-handed circular polarization antenna 6a and a left-handed circular polarization antenna 6b are attached to a set box 5 or the like. For example, the right-handed circular polarization antenna 6a and the left-handed circular polarization antenna 6b are orthogonally arranged. Thus, orthogonal arrangement of these polarizations in the linear polarization antenna and the circular polarization antenna allows any quality in the MIMO transmission to be improved.